SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended
	Mar. 31, 2017 USD ($) trust	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Securitized credit card receivables
Number of trusts to hold securitized credit card receivables | trust	2
Ownership interests in principal amount of trust credit card receivables
Sold to investors via trust-issued securities	$ 23.2		$ 22.7
Retained by Citigroup as trust-issued securities	7.5		7.4
Retained by Citigroup via non-certificated interests	16.2		20.6
Total ownership interests in principal amount of trust credit card receivables	46.9		$ 50.7
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	2.5	$ 0.0
Pay down of maturing notes	$ (2.0)	$ (2.2)